Modern Engineering Thermodynamics

11.10 Compressibility Factor and Generalized Charts 389
4.0
0.105
Compressibility factor, Z = pv/RT
3.0
2.0
T/T c = 1.00
1.05
1.10
1.15
1.20
1.30
0.16
0.30
1.40
1.50
1.60
1.80
2.00
2.50
3.00
3.50
4.00
5.00
6.00
0.110
0.115
8.00
10.00
15.00
0.12
0.13
0.14
0.20
υ/υ c = 0.60
1.0
10 15 20 25
Reduced pressure, p R = p/p c
30
35
40
FIGURE 11.7
The generalized (Nelson-Obert) compressibility chart—high-pressure range. Note that v′ R = v/v′ c = vp c /RT c : (Source: “Nelson-Obert
Compressibility Charts,” adapted courtesy of Professor E. F. Obert, University of Wisconsin, Madison, from Obert, E. F., 1960. Concepts
of Thermodynamics. McGraw-Hill, New York.)
EXAMPLE 11.14
Compressed natural gas (CNG) is essentially methane (CH 4 ). CNG is currently being used as a replacement fuel for gasoline in
some automobiles. This requires replacing the automobile’s gasoline tank with a high-pressure 0.100 m 3 cylinder filled with
CNG. Under normal conditions, the tank pressure is no more than 20.0 MPa when the tank is filled with a maximum of 15.6 kg
of CNG. However, the worst case condition would be if the automobile were consumed by fire and the tank temperature reached
1000.°C. Using the compressibility charts, determine the maximum pressure in the CNG tank at this worst case temperature.
Solution
First, draw a sketch of the system (Figure 11.8).
CNG tank
at 1000.°C
FIGURE 11.8
Example 11.14
The unknown is the maximum pressure in the CNG tank. From Table C.12b, we find the critical state properties of methane to be
T c = 191:1 K and p c = 4:64 MPa
(Continued )